Hydroxyzine‐induced fixed drug eruption and cross‐reaction with oxatomide

causative drug. PT at the lesion is an effective means for diagnosis. 3 However, few reports have implicated such drugs in the de-velopment of FDE, particularly the piperazine derivatives, such as hydroxyzine, cetirizine, and levocetirizine. 1 Bhari et al. 4 reported the case of a patient with an allergy to a drug containing a piperazine ring, who presented a cross- reaction to two other drugs containing piperazine rings. There are no reports on the cross-reactivity between oxatomide and hydroxyzine; however, both share a piperazine ring structure (red circles in Figure 1E,F). Thus, cross- reactivity could appear. In this case, although eszopiclone has a piperazine ring (Figure 1G), no eruption has been reported to date. Therefore, the antigenic determinant in our case was not a piperazine ring. Oxatomide and hydroxyzine share not only a piperazine ring but also two benzene rings via a methine group ( = CH– ; green square in Figure 1E,F). This shared structural group was suggested to become an antigenic determinant and results in cross- reactivity. To the best of our knowledge, such a unique case has not yet been reported, and the possibility of a novel antigenic structural group associated with FDE is proposed in this case, which needs further investigations.